The invention described herein relates to a method for separating isotopes and more specifically to method wherein said isotopes are separated as a result of selective or preferential photochemical decomposition of the desired isotope bearing feedstock material in cryogenic solution. The necessary coincidence in wavelength between the light source emission and the absorption of said isotope bearing species is accomplished by choosing the appropriate cryogenic solvent and not by "tuning" the light source which is commonly done if said source can actually be adjusted in wavelength as can some lasers. Recent success in the separation of D from H in cryogenic solutions of formaldehyde has stimulated interest of the inventors in extending the technique to the separation of isotopes of other light elements. A concurrent vapor-phase isotope separation of carbon isotopes from the low-pressure photolysis of CS.sub.2 with light from an ArF laser, the existence of predissociative states which give rise to well-defined vibronic structure in the region 185-215 nm, and the availability of a convenient and powerful resonance lamp photolysis source suggested to the inventors that CS.sub.2 might be a suitable candidate for a liquid-phase separation. The modest success with this molecule at 206 nm described in detail below was made possible by the utilization of the shift in molecular absorption features resulting from dissolution of the feedstock material, CS.sub.2, in cryogenic solvents, since convenient, high intensity, tunable uv sources in this region have not been developed, especially those with the advantages of continuous-wave operation. It is of importance to note that the iodine resonance lamp was selected because its output corresponds toa region of strong absorption by CS.sub.2. Further, the inventors have discovered that the solvent shift is a general phenomenon and not restricted to CS.sub.2 for a feedstock compound and carbon as the only relevant or possible isotopic species. The invention is a result of a contract with the Department of Energy.
Four publications are known to be relevant to the instant invention. The first, "Carbon and Sulfur Isotope Separation by ArF Laser Irradiation of CS.sub.2," by Loree et al., J. Photochem. 10, 359 (1979), describes a vapor phase separation in which the laser is "tuned" into coincidence with an absorption feature of the desired carbon isotope containing CS.sub.2 molecule. Although the photolysis products are almost certainly the same as those for the instant invention, and the process presented therein successful, the paper does not teach the obtaining of the desired coincidence between the light source and the absorptions of the CS.sub.2 molecules containing the sought-after isotope by shifting the molecular spectrum of said molecules to coincide with a fixed wavelength light source by the use of cryogenic solvents.
Patent application Ser. No. 839,238 (Laser-Induced Separation of Hydrogen Isotopes in the Liquid Phase) teaches the cryogenic isotope separation technique of the instant invention but again does not disclose the use of the solvent shift of the molecular spectrum to obtain the required coincidence for isotopically selective photochemistry. The light sources therein described either produce radiation at a desired wavelength or can be "tuned" to such a wavelength.
The third publication, "Quantitative Detection of Trace Impurities in Gases by Infrared Spectrometry of Cryogenic Solutions" by Freund et al., Analyt. Chem. 50, 1260 (1978), discloses that there is indeed a shift in the molecular spectrum as a function of solvent, but neither characterizes said shift nor teaches a photochemical application made possible because of a coincidence becoming available as a result of said shift.
The final work, found in Advances in Infrared and Raman Spectroscopy, Vol. II (Ed. by R. J. H. Clark and R. E. Hester, Heyden, 1976), page 50 mentions the "fine tuning" of absorption frequencies of solutes by appropriate choice of the host in matrix isolation studies, but does not teach the corresponding effect in cryogenic solutions, nor its application to the separation of isotopes.